[C-libcurl] Fix build error when a struct includes a pointer pointing to itself

Created by: ityuhui

[C-libcurl] Fix build error when a struct includes a pointer pointing to itself

After a C client is generated by the C-libcurl OpenAPI-generator, a struct may include a pointer pointing to itself, e.g.

typedef struct v1_json_schema_props_t {
    ...
    v1_json_schema_props_t *not;  
    ...
} v1_json_schema_props_t;

C compiler cannot recoginze v1_json_schema_props_t *not; so the building will fail.

We need change the template code, let the generated code like below:

typedef struct v1_json_schema_props_t {
    ...
    struct v1_json_schema_props_t *not; //model
    ...
} v1_json_schema_props_t;

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the master: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

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